1. Field of the Invention
The present invention relates to an implantable, modular tissue stimulator comprising (a) a master controller including a micro-controller, non-volatile memory, circuitry for receiving signals down-linked by external devices representing commands or program data and circuitry for up-linking signals representing stored data or commands to external devices; (b) two or more I/O modules each having two or more stimulating electrodes, circuitry for delivering electrical pulses of different amplitude, pulse width and rate to each electrode, and circuitry for powering the circuitry driving each electrode from an isolated power source; and, (c) a bidirectional bus carrying data and controlling signals between the master controller and the I/O modules.
2. Description of the Prior Art
The concept of using implantable, electrically operated tissue stimulators for treating specific diseases or physical disorders is well known. Some examples are cardiac pacemakers which restore a sick human heart to a normal rhythm, neural stimulators which control nerve or brain response (such as pain or epileptic seizures), and cardioverter defibrillators which, upon sensing a fibrillation episode, automatically delivers an electric shock to the heart to restore a normal rhythm.
The use of implantable, electrically operated neural stimulator systems has been well established for a number of years, especially for the control of nerve or brain response to treat intractable pain, epileptic seizures and tremors as a result of Parkinson disease.
An example of a prior art neural stimulator system is an RF coupled device comprising an external RF transmitter and a surgically implanted receiver whereby RF energy is coupled into the receiver to power the neural stimulator. The RF transmitter generates a radio frequency carrier which is modulated twice for each stimulus pulse, first to designate to the receiver which electrodes are to be used to deliver the stimulus pulse and their respective polarity, and secondly, the amplitude of the carrier is modulated for a period corresponding to the stimulus pulse width, the peak modulated voltage representing the pulse amplitude.
The amplitude of the stimulation pulse delivered by the receiver is proportional to the peak value of the RF signal received during the stimulation portion of the carrier wave form. The frequency of stimulation is controlled by the RF transmitter by simply adjusting the repetition rate of the modulation.
For some clinical indications, electrical isolation between separate stimulation channels is required so that when one channel delivers a stimulus pulse the current flow is confined to the active electrodes within that channel and do not traverse to electrodes in another channel.
For example, electrical isolation may be required for treating patients feeling pain in more than one area of their bodies. Such patients benefit from a single implanted stimulator having two or more channels, each channel delivering a different stimulation schedule, and each schedule being designed to provide pain relief in a specific area of the patient's body.
However, there are other clinical applications in which current traversing between electrodes in different channels is desirable, such as when trying to recruit specific nerve fibers by forming current vectors between two or more stimulation channels whereby the resulting electric field can be steered and focused with greater precision than is otherwise possible with a single stimulation channel.
In summary, it is a desirable option to be able to non-invasively program the device to stimulate in "isolation" mode when the electric current needs to be confined to pre-selected electrodes within the channel generating said current, or alternatively in "combination" mode when a steerable electric field is required to achieve effective therapy.
RF coupled neuro modulation systems are easily configured to multiple channels where each channel must be programmed to a different amplitude than other channel(s) but with electrical isolation between the two channels. Since inductors are employed in the implanted RF stimulator to gather the RF energy which powers the stimulator, one receiving inductor per channel can be used to provide an independent power supply for each channel in order to achieve total isolation between channels. Furthermore, independent frequency and pulse width can be achieved easily in the RF coupled stimulator by simply alternatively modulating the carrier at two (or more) different frequencies, each frequency value designating the pulse width and rate for a particular channel.
In the case of a self-powered stimulator employing a single battery as the power source, it is much more difficult to design a multiple channel stimulator with total isolation between channels, when each channel stimulates at a different amplitude, pulse width and rate. For reasons of volumetric efficiency and manufacturing cost, it is impractical to design a multichannel, self-powered stimulator employing several batteries (one for each channel) in order to achieve the necessary isolation. Furthermore, if one battery per channel were used, the resulting stimulation system would only operate in the "isolation" mode and the "combination" option would not be available.
Another deficiency often encountered with prior art neural stimulators, is that constant voltage pulses are used to stimulate nerve tissue. In a constant voltage stimulation the voltage amplitude remains unchanged as the impedance at the electrode/tissue interface varies. If a multiple channel neural stimulator generating constant voltage pulses is employed in the "combination" mode to steer and focus the electric field to recruit specific nerve tissue, the location of the electric field will migrate as a result of the impedance variations, resulting in inefficient therapy.
Another deficiency often encountered with prior art neural stimulators, is that delivered amplitude varies as a function of stimulation pulse width and electrode impedance, due to losses in the transistors used to deliver the stimulation pulses to the electrodes. Pulse amplitude is probably the most critical stimulation parameter for pain control, since the amplitude has to be maintained within a narrow band which produces a sensation of paresthesia. If the amplitude is allowed to decrease as lead impedance or pulse width are increased, ineffective therapy may result.
Another deficiency found in prior art, multiple-channel neural stimulators, is that all electrodes within a given channel receive the same pulse amplitude, width and rate making steering of the electric field a difficult, imprecise and time consuming clinical procedure.
Another deficiency found in prior art neural stimulators, is that patients wearing spinal cord stimulators often experience periods of under stimulation or over stimulation due to postural changes, frequently requiring reprogramming of the pulse amplitude until paresthesia is reestablished.
Another deficiency found in prior art neural stimulators has to do with their conventional hardware architecture which can only control a limited number of channels and electrodes, limiting therapy effectiveness.
Still another deficiency found in prior art neural stimulators, is the lack of capability to sense and respond to biological signals. The effectiveness of neural stimulators may be improved if delivery of medical therapy is started or stopped upon sensing a specific biological signal.